The invention relates to radio receivers of a type suitable for receiving broadcast digital television signals.
Television signal receivers for receiving digital television (DTV) signals that have been proposed by the Grand Alliance, a group of DTV proponents including Zenith Electronics Corporation, use plural-conversion radio receivers. During the first detection procedure in these plural-conversion radio receivers, DTV signal in a selected one of the ultra-high-frequency (UHF) channels is up-converted in frequency to first intermediate-frequency signal in a first intermediate-frequency band centered at 920 MHz. This puts the image frequencies above 1 GHz, making them easy to reject by fixed-tuned front-end filtering. The upconverted DTV signals are then amplified in a first intermediate-frequency amplifier that uses ceramic resonators for tuning. The resulting amplified first intermediate-frequency signal is then down-converted in frequency by mixing it with 876 MHz local oscillations, resulting in a second intermediate-frequency signal in a second intermediate-frequency band 6 MHz wide centered at 44 MHz. The overall amplitude and phase characteristics of the receiver are controlled using a surface-acoustic-wave (SAW) filter for selecting the second intermediate-frequency band. This second intermediate-frequency signal, as selected by the SAW filter, is then amplified in a second intermediate-frequency amplifier. The response of the second I-F amplifier is then synchrodyned to baseband. This synchrodyning procedure can be a direct one in which the response of the second I-F amplifier is synchronously detected at the frequency of the data carrier in the second I-F band.
Alternatively, as described by C. B. Patel and A. L. R. Limberg in U.S. Pat. No. 5,479,449 issued Dec. 26, 1995 and entitled xe2x80x9cDIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVERxe2x80x9d, this synchrodyning procedure can proceed by stages. A final I-F signal referred to as a xe2x80x9clow-bandxe2x80x9d final I-F signal is used that has its uppermost frequency in the HF band, which extends from 3 to 30 MHz. U.S. Pat. No. 5,479,449 describes the response of the second I-F amplifier being downconverted to a third and final intermediate-frequency band somewhere in the one to eight megahertz frequency range, rather than being downconverted to baseband, and then synchronously detected at the frequency of the data carrier in the final I-F band. This alternative synchrodyning procedure is preferable if synchronous detection is done in the digital regime, rather than the analog regime, since the sampling rates required in analog-to-digital conversion can be lowered sufficiently to make such conversion practical with currently available technology. Analog-to-digital conversion of second I-F signals in a second I-F band 6 MHz wide centered at 44 MHz is difficult to do with the requisite 10- to 12-bit resolution, and analog-to-digital converters capable of such performance currently are considered to be too expensive to include in a mass-market consumer product. However, the two-step downconversion from the UHF first I-F band to the final I-F band somewhere in the one to eight megahertz frequency range, still involves some expense.
Television receivers for receiving analog television signals transmitted according to the National Television Systems Committee (NTSC) broadcasting standard are almost invariably single-conversion receivers. These receivers have generally used an I-F band 6 MHz wide centered at 44 MHz. When UHF television broadcasting was instituted the 21.25-27.25 MHz I-F band commonly used in VHF-only television receivers exhibited image rejection problems when UHF signals were being received. In order to avoid image frequencies falling into the reception band a higher intermediate frequency was necessary, and centering of the I-F band at 44 MHz became standard. Since DTV receivers use plural-conversion to allow UHF transmitters to broadcast with less frequency separation between their respective transmission channels, the use of the VHF I-F to avoid problems with images is not necessary.
Accordingly, a single-step downconversion from the UHF first I-F signal to a low-band final I-F signal should be feasible, the inventors realized. This low-band final I-F signal can have its uppermost frequency below 10 megahertz or so as described in U.S. Pat. No. 5,479,449. However, advances in analog-to-digital conversion now make digitization with 10-bit to 12-bit resolution sampling rates somewhat over 80 megahertz feasible, so the final I-F signal can have its uppermost frequency further up in the HF range. The final I-F signal can be in the 21.25-27.25 MHz I-F band used in VHF-only television receivers of the past, for example. Locating the final I-F signal at higher frequencies in the HF band facilitates the use of surface-acoustic-wave (SAW) filtering in the amplifier for the final I-F signal, the inventors note.
The Advanced Television Systems Committee (ATSC) standard for digital television broadcasting uses vestigial-sideband amplitude modulation of a carrier 310 kilohertz above the lower edge of the six-megahertz-wide transmission channels. Eight-level (3 bits/symbol) one-dimensional-constellation symbol coding at 10,762,238 symbols per second rate is specified for over-the-air transmission. The inventors note that a carrier frequency of 21.52 MHz, as could repose in a 21.20 to 27.20 MHz DTV final I-F signal band or in a 15.83 to 21.83 MHz DTV final I-F signal band, is harmonically related to this symbol rate. Further, the inventors discern that sampling at 4xc3x9721.52=86.08 million samples per second more than adequately samples the 21.52 MHz carrier frequency, to facilitate synchronizing a carrier wave regenerated in the receiver to that carrier using an automatic frequency and phase control (AFPC) feedback loop. The 86 MHz analog-to-digital converters can be used as the demodulators to baseband, by selecting alternate samples and then inverting every other one of the selected samples.
A plural-conversion digital television signal receiver that embodies the invention upconverts digital television signal from a selected television channel to a UHF initial I-F signal. After image-rejection filtering there is a single-step downconversion of the initial UHF first I-F signal to a low-band final I-F signal, the uppermost frequency of which is in the high-frequency band. The low-band final I-F signal is digitized and demodulated in the digital regime to generate a baseband signal.